This invention relates generally to apparatus for upgrading existing gas or oil fired furnaces and, more particularly, to combustion gas heat recovery apparatus for use with such existing furnaces.
In these times of ever increasing fuel prices, great effort has been expended to extract a maximum amount of heat from a given amount of fuel. These efforts have led to new furnace designs which are extremely efficient, with many designs boasting efficiencies around the 90 percentile mark. Unfortunately, in the recent past when fuel costs were not as severe, furnace design was more devoted to decreasing the cost and size of the central heating units, and it was a common practice to minimize the heat exchanger. The frequent result was that a substantial amount of the generated heat was allowed to wastefully flow up the chimney. Presently, many such inefficient furnaces are in use and can be expected to perform for many more years of effective, even though inefficient, operation.
While these inefficient furnaces could be replaced by the newer, high efficiency designs, the costs involved are prohibitive. To improve the efficiencies of these furnaces, a variety of combustion gas or flue gas heat recovery systems have been developed. One common variety of flue gas heat recovery system involves the installation of a heat exchanger into a flue pipe which interconnects the furnace to a chimney such that the hot flue gases flow through a sealed passage of the heat exchanger. Ambient air is drawn through the heat exchanger to extract heat from the flue gases with the heated air remaining in the room which contains the furnace. An example of such a flue gas heat recovery system is disclosed in U.S. Pat. No. 3,813,039.
Unfortunately, the furnace room normally is not where additional heat recovered from the flue gases is required. Systems have been developed which circulate the recovered heat throughout the structure being served by the furnace. Some of these systems directly pass flue gases to a heat exchanger connected to preheat cold air which is drawn into a furnace and then further heated within the furnace as exemplified by U.S. Pat. Nos. 4,176,787 and 4,241,874. Other systems utilize a heat conveying fluid which is contained within a sealed system. The fluid is heated in a heat exchanger in intimate contact with the furnace flue pipe or over which heated combustion gases are passed, with the heated fluid being circulated to a second heat exchanger positioned within the cold air return path of the furnace where the heat is extracted from the fluid. Systems of this type are exemplified by U.S. Pat. Nos. 4,392,610 and 4,401,261.
While the noted systems provide heat recovery from the hot combustion or flue gases of existing furnaces, their efficiencies do not approach the high efficiencies of the improved furnace designs which are now available. It is thus apparent that the need exists for a combustion gas heat recovery system to upgrade existing gas or oil fired furnaces to improve the efficiencies of the existing furnaces to rival those of the newer furnace designs and which can be installed at a fraction of the cost of replacing the existing furnaces.